The present invention relates generally to sensing techniques for human-computer interface devices.
Using an interface device, a user can interact with an environment displayed by a computer system to perform functions and tasks on the computer, such as playing a game, experiencing a simulation or virtual reality environment, using a computer aided design system, operating a graphical user interface (GUI), or otherwise influencing events or images depicted on the screen. Common human-computer interface devices used for such interaction include a joystick, mouse, trackball, steering wheel, stylus, tablet, pressure-sensitive ball, or the like, that is connected to the computer system controlling the displayed environment.
In some interface devices, haptic feedback is also provided to the user, also known as “force feedback.” These types of interface devices can provide physical sensations which are felt by the user manipulating the physical object of the interface device. For example, the Force-FX joystick controller from CH Products, Inc. or the Wingman Force joystick from Logitech may be connected to a computer and provides forces to a user of the controller. Other systems might use a force feedback mouse controller. One or more motors or other actuators are used in the device and are connected to the controlling computer system. The computer system controls forces on the force feedback device in conjunction and coordinated with displayed events and interactions on the host by sending control signals or commands to the force feedback device and the actuators.
In both force feedback devices and non-force feedback devices, it is important to accurately sense the position of the manipulandum moved by the user so that interactions displayed on the screen are accurately portrayed. Furthermore, the determination of forces in force feedback devices often depends on the position of the manipulandum so that accurate sensed position is critical for realistic force feedback.
Thus, all interface devices require some sort of position sensor (or sensors) in order to read the input position of the manipulandum (“end effector”). Sometimes the position sensors are connected directly to the end effector itself. However, in other cases the sensor is actually connected through a mechanical transmission to the end effector. For example, a gear transmission, capstan drive (cable) transmission, or belt drive transmission can be used. The purpose of the transmission, as far as the position sensor is concerned, is to amplify the range of motion that the sensor can detect and allow the use of lower resolution sensors, which are typically less expensive. A mechanical transmission may also be used to amplify forces output by the actuators of the force feedback device. The transmission for a sensor can be the same transmission as an actuator uses; several different joystick devices use this method, such as the device described in copending patent application Ser. No. 09/138,304, which provides a sensor connected to an actuator shaft. In other devices, an independent mechanical transmission dedicated to the sensor can be used.
In either of these cases, there is the possibility of position slip occurring in the transmission, where the end effector moves some distance without the sensor detecting that movement or inaccurately detecting that movement; e.g., a gear skips teeth, a belt or cable slips on a pulley, etc. When a relative position sensor is used in the device, such as a quadrature encoder, this position slip should be detected or the device will be unable to determine an accurate position for the end effector when slip occurs. The problem is amplified for force feedback devices because they often operate in a limited workspace and depend on the ability to determine the absolute device position in the limited workspace. Furthermore, some forces output by the device are based on an accurate position of the manipulandum, so that the forces can become discontinuous or distorted if position slip occurs.